The present invention relates to a device inside a glass tube in a tubular fluorescent lamp, where the tubular fluorescent lamp comprises an electrode mounted inside the glass tube said electrode being at least partially surrounded by an electrode cover mounted inside the glass tube.
JP 56134468 (Patent abstracts of Japan Vol. 006012, Jan. 23, 1982) discloses as previously known a tubular fluorescent lamp which comprises a glass tube and an electrode, where the electrode in its mounting position is placed at a distance from the inside of the wall of the glass tube.
EP 0 555 619 A1 discloses as previously known a tubular fluorescent lamp which comprises a glass tube and an electrode and, placed in front of the electrode a plate made of an electrically insulating material, where the electrode in its mounting position is disposed at a distance from the inside of the wall of the glass tube.
WO 81/01344 discloses as previously known a tubular fluorescent lamp which comprises a glass tube and an electrode that is surrounded by an electrode cover which is made of an electrically conducting material and is not electrically connected with the electrode. The electrode cover consists of a can-shaped casing with an aperture made in its bottom end for the insertion of the electrode into the interior of the can. The free end of the can is closed by a plate provided with a central hole and made of an electrically insulating material.
Tubular fluorescent lamps of the above mentioned kind are provided with electrodes, that operate alternating as cathodes and anodes, where the cathode function is the critical one with respect to service life, burning hours and product safety.
The tubular fluorescent lamp market of today is dominated by tubular fluorescent lamps that have electrodes of the so-called hot cathode type. This electrode type is provided with special emitter material which has the ability to emit electrons at relatively low temperatures and relatively small energy supply. The energy necessary for the electron emission is supplied partly through electric heating of the coil of the electrode, which may be a tungsten coil, partly from the kinetic energy of incoming gas ions (cathode function) and electrons (anode function).
Cathode voltage drop and anode voltage drop is in a working tubular fluorescent lamp of the order of magnitude of 10V, and the hottest spot on the tubular fluorescent lamp glass, that is on the glass tube, is in the vicinity of the electrodes, still without reaching such values that may jeopardize safety.
When an electrode has completely, or almost completely, lost its emitter material, the cathode voltage drop increases substantially, which means that both the number of incoming gas ions and their kinetic energy increases substantially, which leads to a dramatic increase in heat release in the actual electrode region.
As far as can be assessed, the heat energy is concentrated initially to the coil. If it melts down quickly and loses its connection with the power supply, the heat energy will be concentrated to the current supply wires which then may melt down and cause melted metal to drip down on the inside of the glass tube. In tubular fluorescent lamps according to JP 56134468 and EP 0 555 619 A1, that is, tubular fluorescent lamps that lack an electrode cover, there is nothing to prevent this. In tubular fluorescent lamps according to WO 81/01344, that is, tubular fluorescent lamps that have an electrode cover which at least partially is placed between the coil and the inside of the glass tube seen vertically when the tubular fluorescent lamp is mounted in its working position, which means horizontally or at an angle to the horizontal plane, these drops will be collected by the electrode cover, at least if you have a relatively large electrode cover as shown in this document, which cover consequently may stop the drops from reaching the inside of the surface of the glass tube.
If the coil remains intact or remains essentially in the original position for several minutes, the electrode cover itself, in those cases where there is one, will be significantly heated up. Then, when conduction heat from the electrode cover makes the glass in the sealing area soft, the electrode cover may bend down due to gravity and come into contact with the inside of the surface of the glass tube.
A crack in the glass tube may consequently be caused by melted metal drops or the hot electrode cover coming into contact with the inside of the glass tube surface. These cracks may cause the tubular fluorescent lamp to break and possibly fall out of its fittings. This phenomenon is well known under the term xe2x80x9cSafety at end of lifexe2x80x9d. Security aspects in connection with the burning out of tubular fluorescent lamps are dealt with in European and international standards concerning tubular fluorescent lamps and their operating components, under the section xe2x80x9cAbnormal conditionsxe2x80x9d.
Electrical devices that are built into tubular fluorescent lamp operating components of high frequency type with the object of preventing this increase in heat generation in the electrode region are previously known.
The object of the invention is to prevent the tubular fluorescent lamp from falling out of its fittings at the end of its life.
This is achieved with a tubular fluorescent lamp, comprising:
(a) a hollow, elongated glass tube, having at least one end opening,
(b) at least one tube end sealing assembly mounted in and sealing said at least one end opening and including: a base, contact pins extending out from said base, an electrode attached to said base, current supply wires connecting said electrode with said contact pins through said base, an electrode cover surrounding said electrode,
(c) a spacer between the electrode cover and the adjacent inner surface of said glass tube, whereby direct contact is prevented between the electrode cover and the inner surface of the glass tube.
By using tubular fluorescent lamps according to the invention which prevent direct contact between the electrode cover and the inside of the glass tube, cracks in the glass tube in connection with burning out of the tubular fluorescent lamps caused by the hot electrode cover coming into contact with the inside of the glass tube are avoided. At the same time, the function of the electrode cover to prevent molten metal drops from the coil from dripping down on the inside of the glass tube surface, which may cause cracks in the glass tube, is maintained. These cracks may cause the tubular fluorescent lamp to break off and fall out of its fittings.